1. Field of the Invention
The present invention relates to a thermal print head for thermal printing.
2. Description of the Related Art
The thermal print head which has such a structure as shown in FIGS. 8 and 9 is well-known. In FIGS. 8 and 9, reference numeral 1 denotes a substrate made of ceramics or the like. Swelled portions 2 having a certain interval relative to its adjacent ones and each being made of SiO.sub.2, SiN.sub.1 or the like are formed on the substrate 1 in the width direction thereof by the CVD (Chemical Vapor Deposition) method. Thermal resistive thin films 3 are formed on the substrate 1 and on each of the swelled portions 2. Electrodes 4 and 5 are formed on each of the thermal resistive thin films 3 except the center portion of the film 3. The electrodes are selective ones and the other electrode 5 is a common one. The common electrode 5 is connected to ground potential. The electrodes 4, 5, the center portions of the thermal resistive thin films 3, and the exposed portion of the substrate 1 are coated by an insulating protection film 6 (not shown in FIG. 9).
In the case of this thermal print head, printing current supplied selectively responsive to printing data is applied to the common electrode 5 through the center portions of the thermal resistive thin films 3. Center portions of the thermal resistive thin films 3 which are not covered by the electrodes are selectively heated by printing current. These center portions or heated portions of the thermal resistive thin films 3 are kept higher by the swelled portions 2 than the protection film 6 on the substrate 1, so that they can be reliably contacted with a sheet of printed paper to clearly print thereon.
In the case of the thermal print head having the above-described structure, however, it is desirable that each of the swelled portions 2 is formed to have a smooth slope from the bottom to the top thereof and made sufficiently higher than the other portion of the substrate 1. An extremely long processing time is needed to form each of the swelled portions 2 with a thickness of the order of microns according to the CVD method. Further, it is quite difficult to smoothly tilt the rim portion of each of the swelled portions 2 according to the thin film forming manner. Furthermore, the cost becomes disadvantageously high because the substrate 1 must be made of expensive material.
In order to solve these problems, there has been discussed a manner of print-forming the thermal resistive films on a flexible substrate by carbon ink. The swelling of the heated portions is carried out in this case by forming the swelled portions at intended portions of a support member mounted on a hard substrate and laminating the flexible substrate onto the support member while corresponding the thermal resistive thin films to the swelled portions of the support member. According to this print-forming method, however, the difference of film thicknesses of the thermal resistive thin films is larger, thereby making their temperatures more uneven when they are heated than that according to the CVD method. In this method, further, there is a difficulty having a uniformity of the heights for all the swelled portions. Therefore, this method can make the cost of printing lower but the quality of printing worse.